The present invention relates to an ink-jet printer for performing printing by spitting ink on a paper sheet held by a rotary drum.
In recent years, high-performance, low-cost personal computers are easily accessible, and are spreading rapidly. With this rapid spread, demands for color printers also increasing. At present, a serial ink-jet printer is known as a color printer suitable for personal use.
A general serial ink-jet printer 500 will now be described with reference to FIGS. 1 and 2. The ink-jet printer 500 is constituted to, while moving a print head 501 across a paper sheet M in a main scanning direction X, spit ink on the paper sheet M to perform printing of one line, and supply the paper sheet M at a predetermined pitch along a paper feed path set in a sub-scanning direction Y perpendicular to the main scanning direction X every printing of one line. The print head 501 has first to fourth nozzle units 502 for printing an image in yellow, magenta, cyan, and black. Each nozzle unit 502 has a plurality of ink-jet nozzles aligned in the sub-scanning direction Y at a pitch P required to obtain a predetermined resolution. For example, for a resolution of 300 dots/inch, the nozzle pitch P is set at 1/300 inch. These ink-jet nozzles are oriented to the paper sheet M fed in the sub-scanning direction Y beneath the nozzles, and used to eject ink having a corresponding color which is supplied from first and second detachable ink cassettes 504, and used to put the ink on the paper sheet M. In general, yellow, magenta, and cyan inks are stored in the first ink cassette 504, and a black ink is stored in the second ink cassette 504. The print head 501 is mounted on a carrier 505 driven by a moving mechanism 510. The moving mechanism 510 includes a guide rod 511 extending in the main scanning direction X, a pair of sprockets 512 arranged near the two ends of the guide rod 511, a timing belt 513 disposed between the sprockets 512, and a stepping motor for rotating the timing belt 513 via the sprockets 512 at a predetermined speed. The carrier 505 is fixed to part of the timing belt 513, and set to be reciprocative along the guide rod 511 according to the rotational direction of the timing belt 513.
In the ink-jet printer 500, the paper sheet M is fed in the sub-scanning direction Y along the paper feed path until it opposes the print head 501, and the carrier 505 moves forward along the guide rod 511 in the main scanning direction X. Meanwhile, the ink-jet nozzles of the first to fourth nozzle units 502 spit yellow, magenta, cyan, and black inks on the paper sheet M to perform color printing of one line. After the printing, the carrier 505 moves backward, and the paper sheet M is fed by a distance corresponding to the height of one line. By repeatedly performing this printing operation, an image of characters, symbols, or graphics is color-printed on the entire paper sheet M. Upon completion of the printing, the paper sheet M is further fed in the sub-scanning direction Y and discharged to a discharge space S.
As another color printer, for example, a laser printer 600 shown in FIG. 3 has conventionally been known. The laser printer 600 has processing portions 610Y, 610M, 610C, and 610B aligned along a paper feed path within a main body case 601 in order to perform printing in yellow, magenta, cyan, and black. The paper sheet M is fed by a feed mechanism 620 so as to sequentially pass through the processing portions 610Y, 610M, 610C, and 610B. Each of the processing portions 610Y, 610M, 610C, and 610B includes a photosensitive drum 611 which is set perpendicular to the paper feed path and rotates in one direction, and a laser-beam emitter, a charger, a developing unit, a transfer unit, a waste toner collecting unit, and a discharger which are arranged around the photosensitive drum 611. Each of the processing portions 610Y, 610M, 610C, and 610B forms an electrostatic latent image on the photosensitive drum 611 by a laser beam, supplies a corresponding color toner to be adhered to the latent image as a toner image, transfers this toner image to the paper sheet M fed along the paper feed path, and fixes the toner image using a thermal fixing unit arranged on the paper feed path.
The color printing speed of the ink-jet printer 500 is very low. The printer 500 is constituted to perform printing on the entire paper sheet M fed in the sub-scanning direction Y by the print head 501 which reciprocates in the main scanning direction X. For this reason, a printing time of about 5 to 10 minutes is required for printing of an A4-size paper sheet. When high-resolution color printing is performed for, e.g., an A4-size paper sheet, a printing time of about 5 to 10 minutes is necessary for one paper sheet. If, e.g., the ink cassette 504 is downsized to reduce the load weight of the carrier 505, the printing speed can be increased. However, this results in a decrease in ink exchange interval. Since the ink cassette 504 must have a capacity that covers printing of at least 200 paper sheets by one ink exchange, it is difficult to remarkably improve the printing speed by downsizing the ink cassette 504. Even if the ink cassette 504 is suppressed to the minimum capacity, large noise is generated every time the carrier 505 reciprocates.
On the other hand, since the laser printer 600 is constituted to transfer a toner image adhered to each photosensitive drum 611 to the paper sheet M, no large noise is generated unlike the ink-jet printer 500 in which an ink is spitted on the paper sheet M while reciprocating the carrier. When high-resolution color printing is performed for an A4-size paper sheet, a printing time of about 15 to 20 seconds is required for one paper sheet. That is, printing of four paper sheets can be performed within 1 minute. This color printing speed is much higher than that of the ink-jet printer 500 though the color printing speed is limited by the drum rotation speed set to repeatedly use the photosensitive drum by performing residual toner cleaning, discharging, recharging, and the like for the photosensitive peripheral surface of the drum. This color printing speed does not pose any problem in personal use such as printing of some paper sheets in a postcard size. However, this color printing speed is not satisfactory in office use requiring printing of many paper sheets in larger sizes. Since the high-cost processing portions 610Y, 610M, 610C, and 610B are aligned along the paper feed path, the size and cost of the laser printer 600 inevitably increase. Therefore, the laser printer 600 is not spread as a color printer for personal use.